The present invention relates to multilayer coatings, particularly to Mo-Si multilayer reflective coatings, and more particularly to Mo-Si multilayer coatings having a near normal incidence reflectance of .gtoreq.65% in the extreme ultraviolet wavelength region and a magnitude of residual stress of not greater than about 100 MPa and to the method for fabricating same.
Multilayer structures composed of alternating thin layers of materials with vastly different optical properties have proven effective as high-reflectance coatings for various applications. The Mo-Si system which gives a high reflectance (.about.60%) for certain wavelengths, such as in the 13.0-13.5 wavelength region, is of particular technological importance due to its application to high-resolution, multiple-reflection imaging systems now being developed for projection lithography.
Extreme-ultraviolet (EUV) lithography systems require several high precision optics coated with reflective multilayers. To obtain sufficient throughput and image quality, these multilayer coatings must simultaneously have high reflectance (R.gtoreq.65%) and low magnitude of residual stress (less than 100 MPa). The multilayer coatings generally consist of alternating layers of molybdenum (Mo) and silicon (Si).
There is a strong commercial driving force for increased miniaturization in electronic devices and, hence, an EUV lithography tool has significant commercial potential. The performance of an EUV lithography tool is key to its implementation over other competing technologies, and high film stresses and low EUV reflectances degrade the performance of a EUV lithography tool.
Over the past decade or so numerous publications described the dependency of EUV reflectance of Mo-Si multilayer mirrors or optics on their fabrication parameters. However, the number of publications addressing the problem of stress in Mo-Si multilayers designed for high reflectance in the EUV wavelength is relatively small. The later publications describe how the stress of Mo-Si multilayer coatings can be reduced to low levels (&lt;100 MPa) by: 1) post-deposition annealing (see Kola et al, Appl. Phys. Lett. 60,3120 (1992) and Kassner et al, J. Mat. Sci. 31, 2291 (1996); 2) variation of the Mo to Si layer thickness or ratio (see Nguyen et al, in Physics of X-Ray Multilayer Structures, Optical Society of America, Washington, D.C., 1994, Vol. 6, P. 103; Windt et al, J. Appl. Phys. 78,2423 (1995); and Tinone et al, J. Electron Specrosc. Relat. Phenom. 80,461 (1996); and 3) adjustment of the sputter deposition process such as base pressure or target power (see Windt et al and Tinone et al above). Although Kola et al and Windt et al discuss low stress coatings, none of these prior efforts involve the fabrication of a high reflectance (.gtoreq.65%) Mo-Si multilayer coating with a low magnitude of stress (&lt;100 MPa), and no EUV reflection data was shown, except in Kola et al where reflectances around 58% were measured, which is significantly lower than 65%, which is necessary for use in EUV lithography.
Recently, Mo-Si multilayer coatings using a buffer layer between the substrate and the Mo-Si coatings have provided high reflectance and low stress. The non-thermal or a thermal method of producing the high reflectance-low stress Mo-Si multilayer coatings is described and claimed in copending U.S. application Ser. No. 09/027,309, filed Feb. 20, 1998, entitled "A Method To Adjust Multilayer Film Stress Induced Deformation Of Optics", assigned to the same assignee, now U.S. Pat. No. 6,011,646.
The present invention provides a thermal approach to producing Mo-Si multilayer reflective coatings with high reflectance and low stress, which are particularly applicable for use in an EUV lithography tool. The present invention provides a high reflectance-low stress Mo-Si multilayer coating produced by heating the multilayer coating to a given temperature during a given time period after deposition to induce structural changes in the multilayer coating. Mo-Si multilayer coatings of this invention produced using this method have a high reflectance (R.gtoreq.65%) and a low magnitude of residual stress (.ltoreq.100 MPa). It has been verified by experimentation that using the thermal method, low stresses are obtained in Mo-Si multilayer coatings with minimal (.about.1%-3%) loss in reflectance.